Solid state image device with gate electrodes having low resistance and a method of producing the same

ABSTRACT

A solid state image device including: a plurality of photoelectric converting elements arranged in a matrix pattern along a first and a second direction in a semiconductor substrate; a plurality of electric charge transfer regions for receiving electric charges from the photoelectric converting elements and for transferring the electric charges toward the first direction, the electric charge transfer regions being provided adjacent to the plurality of photoelectric converting elements in the semiconductor substrate and extending along the first direction; and a plurality of gate electrodes for applying a voltage to the electric charge transfer regions to transfer the electric charge from the photoelectric converting elements to the electric charge transfer regions, and to transfer the electric charges toward the first direction, each of the gate electrodes having a plurality of transfer electrode portions provided over the electric charge transfer regions and a plurality of clip electrode portions electrically connecting the plurality of the transfer electrode portions to each other along the second direction, the plurality of the clip electrode portions having a greater thickness than that of the plurality of the transfer electrode portions.

This application is a continuation of application Ser. No. 08/288,937filed on Aug. 10, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid state image device and a methodof producing the same, and more particularly to a solid state imagedevice which is driven at a high speed for adapting to HDTV (HighDefinition TV) systems or the like, and a method of producing the same.

2. Description of the Related Art

An experimental TV program for a HDTV system has been broadcast inrecent years, and there has been an increase in demand for promptlycommercializing the devices for HDTV systems. An HDTV camera is one ofthose devices.

Since HDTV cameras commercially available today use an image tube, theyhave the problems due to these image tubes. Specifically, the image tubetends to break when it is subjected to a large external force, or whenit is vibrated. It is also difficult to produce a smaller image tube.Thus, the development of an HDTV camera employing a solid state imagedevice which can overcome these problems is desirable. Becauseconventional solid state image devices have the following problems, theyare not suitable for HDTV cameras.

FIG. 9 shows a partial plan view of a conventional solid state imagedevice 100. In the solid state image device 100, photodiodes 101 forconverting light into electric charges are formed in a matrix patternalong x and y directions in a semiconductor substrate 102. Verticaltransfer regions 103 extending along the y direction are formed betweenthe photodiodes 101 in the semiconductor substrate 102. The regionswhere the photodiode 101 and the vertical transfer region 103 areprovided are referred to as a photosensitive portion and a verticaltransfer portion, respectively. The region between two adjacentphotosensitive portions along the y direction is referred to as a clipportion.

A plurality of first Gate electrodes 104 shown by dotted lines and aplurality of second gate electrodes 105 shown by solid lines, both in acomb shape, are provided between the photodiodes 101 over thesemiconductor substrate 102.

Each of the first gate electrodes 104 includes transfer electrodeportions 106 provided on the vertical transfer regions 103 through aninsulating film, and clip electrode portions 107 for connecting thevertical transfer electrode portions 106 to each other in the xdirection. Similarly, each of the second Gate electrodes 105 includesvertical transfer electrode portions 108 provided on the verticaltransfer regions 103 through an insulating film, and clip electrodeportions 109 for connecting the vertical transfer electrode portions 108to each other in the x direction.

After electric charges are accumulated in photodiodes 101, each of theelectric charges is transferred to the vertical transfer region 103 asindicated with an arrow 110 by a driving pulse voltage applied to thesecond gate electrodes 105. Then, the electric charge is transferred inthe vertical transfer region 103 in the direction indicated by an arrow111 by driving pulse voltages applied to the first gate electrodes 104and the second gate electrodes 105.

To apply the solid state image device 100 for an HDTV camera, it isnecessary that the solid state image device 100 has a large number ofpixels, such as 1.3 or 2.0 million, and is driven at a high speed.Specifically, the electric charges accumulated in the photodiodes 101need to be transferred at a high speed by driving pulse voltages appliedto the first and second gate electrodes 104 and 105.

However, since the size of the solid state image device 100 increases asthe number of the pixels increases, the first and second electrodes 104and 105 become longer, the effect of the resistance of the gateelectrodes themselves become significant. Thus, since the middleportions of the first and second electrodes 104 and 105 are most distantfrom both ends of each electrode, the effect of the resistances of thegate electrodes to which driving pulse voltages are applied issignificant in these portions. Therefore, the pulse width of a drivingpulse voltage decreases in the middle portions of the gate electrodes,and the maximum amount of electric charge transfer is disadvantageouslyreduced.

Conventional gate electrodes are made integrally by forming polysiliconfilms, each of which has a uniform film thickness, on a semiconductorsubstrate, and etching them into a predetermined shape. Morespecifically, after the deposition of a polysilicon film 121 having athickness of about 450 nm on an insulating film 120 as shown in FIG.10A, a resist pattern 122 is formed on the polysilicon film 121 as shownin FIG. 10B. Then, the polysilicon film 121 is etched using the resistpattern 122 as a mask, and the first gate electrodes 104 are formed byremoving the resist pattern 122. As a result, the first gate electrodes104 of the conventional art having a thickness of 450 nm is formed onthe entire region which has been subjected to the formation steps. Sincethe polysilicon has a high resistance, the problem described above issignificant.

SUMMARY OF THE INVENTION

The solid state image device of this invention includes: a plurality ofphotoelectric converting elements arranged in a matrix pattern along afirst and a second direction in a semiconductor substrate; a pluralityof electric charge transfer regions for receiving electric charges fromthe photoelectric converting elements and for transferring the electriccharges toward the first direction, the electric charge transfer regionsbeing provided adjacent to the plurality of photoelectric convertingelements in the semi-conductor substrate and extending along the firstdirection; and a plurality of gate electrodes for applying a voltage tothe electric charge transfer regions to transfer the electric chargefrom the photoelectric converting elements to the electric chargetransfer regions, and to transfer the electric charges toward the firstdirection, each of the gate electrodes having a plurality of transferelectrode portions provided over the electric charge transfer regionsand a plurality of clip electrode portions electrically connecting theplurality of the transfer electrode portions to each other along thesecond direction, the plurality of the clip electrode portions having agreater thickness than that of the plurality of the transfer electrodeportions.

In one embodiment of the invention, the solid state image device furtherincludes light shielding films, provided over only the electric chargetransfer regions, for shielding the electric charge transfer regionsfrom incident light.

In another embodiment of the invention, the gate electrodes are made ofpolysilicon.

According to another aspect of the invention, a method of producing asolid state image device having a plurality of photoelectric convertingelements arranged in a matrix pattern along a first and a seconddirections in a semiconductor substrate is provided. The method includesthe steps of: forming a plurality of electric charge transfer regionsfor receiving electric charges from the photoelectric convertingelements and for transferring the electric charges toward the firstdirection, the electric charge transfer regions being provided adjacentto the plurality of photoelectric converting elements in thesemiconductor substrate and extending along the first direction; andforming a plurality of gate electrodes for applying a voltage to theelectric charge transfer regions to transfer the electric charge fromthe photoelectric converting elements to the electric charge transferregions, and to transfer the electric charges toward the firstdirection, each of the gate electrodes having a plurality of transferelectrode portions provided over the electric charge transfer regionsand a plurality of clip electrode portions electrically connecting theplurality of the transfer electrode portions to each other along thesecond direction, the plurality of the clip electrode portions having agreater thickness than that of the plurality of the transfer electrodeportions.

In one embodiment of the invention, the step of forming the gateelectrodes includes the steps of: depositing polysilicon on the entiresurface of a semiconductor substrate to form a polysilicon film having apattern for the gate electrodes; forming a resist pattern defining thetransfer electrode portions on a surface of the polysilicon film; andetching the polysilicon film using the resist pattern as a mask.

In the solid state image device of the invention, since the gateelectrodes formed of polysilicon is thicker in the clip portion than inthe other portions, the reduction of the pulse width in the middleportions of the gate electrodes is suppressed even when a driving pulsevoltage is applied on the gate electrodes made of polysilicon from bothsides of a pixel portion.

Moreover, light shielding metal films are formed only in a transferportion while the gate electrodes are made thicker except in thetransfer portion. Therefore, the final thicknesses of the regions exceptthe transfer region do not exceed the thickness of the vertical transferportion. With the structure of the solid state image device of theinvention, the waveform rounding (reduction of the maximum amount ofelectric charge transfer) can be suppressed and driving at a high speedcan be realized without reducing the relative sensitivity when the fvalue of a camera lens is small.

Thus, the invention described herein makes possible the advantage ofproviding a solid state image device and a production method thereofwherein the reduction of the pulse width in the middle portions of thegate electrodes are suppressed even when a driving pulse voltage isapplied on polysilicon electrodes from both sides of a pixel portion.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial plan view of a solid state image device of theinvention.

FIG. 2A illustrates a cross sectional view taken along Line 2A--2A ofthe solid state image device shown in FIG. 1.

FIG. 2B illustrates a cross sectional view taken along Line 2B--2B ofthe solid state image device shown in FIG. 1.

FIG. 2C illustrates a cross sectional view taken along Line 2C--2C ofthe conventional solid state image device shown in FIG. 9.

FIG. 3A shows an example of a gate electrode structure.

FIG. 3B shows a cross sectional view taken along Line 3B--3B in FIG. 3A.

FIG. 3C shows a cross sectional view of a conventional gate electrode.

FIG. 4 is a graphic representation illustrating the relationship betweenthe frame transfer frequency and the rounding of the waveform.

FIGS. 5A and 5B show the differences in incident light paths dependingon the distance between the photodiode and the microlens.

FIG. 6 shows the change of the relative sensitivity with regard to the fvalue of a camera lens.

FIGS. 7A through 7E show formation steps for gate electrodes accordingto the invention.

FIGS. 8A and 8B show other examples of the invention.

FIG. 9 shows a partial plan view of a conventional solid state imagedevice.

FIGS. 10A through 10C show the formation steps for prior art gateelectrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, in part, a schematic plan view of a solid state imagedevice 10 of the invention. Note that for clarity FIG. 1 does not showall of the components necessary for a solid state image device. Thesolid state image device 10 includes a semiconductor substrate 12, andphotodiodes 11 formed in a matrix pattern along the x and y directionsin the semiconductor substrate 12. The photodiodes 11 function asphotoelectric converting elements which receive light and convert itinto electric charges. Vertical transfer regions 13 extending along they direction are formed between the photodiodes 11 in the semiconductorsubstrate 12. Each of the vertical transfer regions 13 is adjacent to aphotodiode 11 and transfers an electric charge generated in thephotodiode 11 toward the y direction. The regions where the photodiode11 and the vertical transfer region 13 are provided are referred to as aphotoelectric transfer region and a vertical transfer region,respectively. The region between two adjacent photoelectric transferportions along the y direction is referred to as a clip portion.

The solid state image device 10 further includes a plurality of firstgate electrodes 14 and a plurality of second gate electrodes 15, both ina comb shape, over the semiconductor substrate

Each of the first gate electrodes 14 includes transfer electrodeportions 16 provided over the vertical transfer regions 13, and clipelectrode portions 17 for connecting the transfer electrode portions 16to each other along the x direction. Similarly, each of the second gateelectrodes 15 includes transfer electrode portions 18 provided over thevertical transfer regions 13, and clip electrode portions 19 forconnecting the transfer electrode portions 16 to each other along the xdirection. The transfer electrode portions 16 partially overlap with thetransfer electrode portions 17.

After electric charges are accumulated in the photodiodes 11, each ofthe electric charges is transferred to the vertical transfer region 13as indicated with an arrow 20 in accordance with driving pulse voltagesapplied to the second gate electrodes 15. Then, the electric charge istransferred in the vertical transfer region 13 in the directionindicated by an arrow 21 by driving pulse voltages applied to the firstgate electrodes 14 and the second gate electrodes 15.

FIGS. 2A and 2B illustrate the cross sectional views taken along Line2A--2A and Line 2B--2B of the solid state image device 10 shown in FIG.1, respectively. The cross section taken along Line 2A-2A shows thephotoelectric transfer portion 23 including the photodiode 11, and avertical transfer portion 24 including the vertical transfer region 13.The cross section taken along Line 2B--2B shows the vertical transferportion 24 and a clip portion 25. A cross section taken along Line2C--2C of the conventional solid state image device 100 illustrated inFIG. 9 is shown for a comparison.

As shown in FIG. 2A, a P⁻ well layer 31 is formed at the surface of thesemiconductor substrate 12 made of N-type silicon. A first diffusionlayer 32 (P layer) is formed in the P⁻ well layer 31. A second diffusionlayer 33 (N layer) and a third diffusion layer 34 (P⁺ layer) are formedon the first diffusion layer 32 (N layer). The first and seconddiffusion layers 32 and 33 constitute the transfer region 13.

Adjacent to the third diffusion layer 34, a fourth diffusion layer 35 (Nlayer) is formed in the P⁻ well layer 31, and the fifth diffusion layer36 (P⁺ layer) is formed on the fourth diffusion layer 35. The fourth andfifth diffusion layers 35 and 36 constitute the photodiode 11.

A first insulating film 37 is formed so as to cover the second diffusionlayer 33 (N layer) and part of the third diffusion layer 34 (P⁺ layer).The transfer electrode portion 16 of the first gate electrode 15 isprovided on the first insulating film 37. The transfer electrode portion16 is made of polysilicon. A second insulating film 38 is formed so asto cover the transfer electrode portion 16.

A third insulating film 39 is formed over the fifth diffusion layer 36(P⁺ layer). The transfer electrode portion 18 of the second gateelectrode 14 is formed on the second insulating film 38 which is formedso as to cover the transfer region 13. The transfer electrode portion 18is made of polysilicon.

A fourth insulating film 40 is formed over the entire semiconductorsubstrate 12. A first light shielding metal 41 is formed on the fourthinsulating film 40 of the vertical transfer portion 24.

Furthermore, a fifth insulating film 42 is formed over the entiresemiconductor substrate 12. A second light shielding metal film 43 isformed on the fifth insulating film 42 of the vertical transfer portion13.

A flattening film 44 is formed with an acrylic resin or the like overthe entire semiconductor substrate 12. A color filter layer 45 is formedon the flattening film 44. A microlens 46, which corresponds to each ofthe photoelectric transfer portions 11 on the color filter layer 45, isprovided.

As shown in FIG. 2B, the structure of the vertical transfer portion 24is the same as that shown in the cross sectional view shown in FIG. 2A.In the clip portion 25, the photodiode 11 is not formed, and only a P⁻well layer 31 is formed at the semiconductor substrate 12. The firstinsulating film 37 is formed on the vertical transfer portion 24 and theclip portion 25. The third insulating film 39 is formed on thephotoelectric transfer portion 11. The transfer electrode portion 16 anda clip electrode portion 17 are formed in the vertical transfer portion24 and the clip portion 25, respectively, through the first insulatingfilm 37. The transfer electrode portion 16 and the clip electrodeportion 17 constitute a part of a continuous first gate electrode 14.The transfer electrode portion 16 and the clip electrode portion 17 aremade of polysilicon, and the clip electrode portion 17 is formed so asto be thicker than (approximately twice as thick as) the transferelectrode portion 16. A second insulating film 38 is formed so as tocover the first gate electrode 14.

Moreover, a transfer electrode portion 18 and the clip electrode portion19 are formed on the second insulating film 38 in the vertical transferportion 24 and the clip portion 25, respectively. The transfer electrodeportion 18 and the clip electrode portion 19 constitute a continuoussecond gate electrode 15. The transfer electrode portion 18 and the clipelectrode portion 19 are made of polysilicon, and the clip electrodeportion 19 is formed so as to be thicker than (approximately twice asthick as) the transfer electrode portion 18. A fourth insulating film 40is formed so as to cover the second gate electrode 15.

A first light shielding metal film 41 is formed on the fourth insulatingfilm 40 in the vertical transfer portion 24. A fifth insulating film 42is further formed over the entire semiconductor substrate 12. A secondlight shielding metal film 43, having a region which is approximately aswide as the vertical transfer portion 24, is formed on the fifthinsulating film 42 on the first light shielding metal film 41.

A flattening film 44 made of an acrylic resin or the like is formed overthe entire semiconductor substrate 12. The surface of the flatteningfilm 44 is flattened. Then, a color filter layer 45 is formed on theflattening film 44 in this order. As is understood from FIGS. 1 and 2B,the microlens 46 is not shown in FIG. 2B. This is because the microlens46 is formed above each of the photodiodes 11 and is not positionedabove the just middle portion between two adjacent photodiodes 11.

In the first and second gate electrodes 104 and 105 of the conventionalsolid state image device 100 shown in FIG. 2C, the transfer electrodeportions 106 and 108 have the same thickness as that of the clipelectrode portions 107 and 109. The second light shielding metal film120 is formed in both of the transfer portion and the clip portion.

As shown in FIGS. 2A and 2B, in the solid state image device 10 of theinvention, the thicknesses of the first and second gate electrodeportions 14 and 15 in the vertical transfer portion 24 are differentfrom those in the clip portion 25. Specifically, the thicknesses of theclip electrode portions 17 and 19 in the clip portion 25 are thickerthan those of the transfer electrode portions 16 and 18 in the verticaltransfer portion 24.

Moreover, in the solid state image device 10 of the invention, the firstand second light shielding metal films 41 and 43 are formed only in thevertical transfer portion 24.

Therefore, the difference in the level between the clip electrodeportions 17 and 19, and the transfer electrode portions 16 and 18, dueto the different film thicknesses can be reduced by providing the firstand second light shielding metal films 41 and 43 only in the verticaltransfer portion 24. Thus, the height from the surface of thesemiconductor substrate 12 to the surface of the flattening film 44 doesnot become too high, and the distance between the microlens 46 and thephotodiode 11 does not become longer than that of conventional elements.

The solid state image device 10 of the invention will be described belowby means of specific examples.

FIG. 3A shows a partial plan view of the surface area of the first gateelectrode 14, made of polysilicon, used in the solid state image device10. FIG. 3B is a cross sectional view taken along 3B--3B in FIG. 3A.

As shown in FIG. 3A, the transfer electrode portion 16 is a rectangulararea of 3.6 μm×2.9(1.3+1.6) μm. The clip electrode portion 17 is arectangular area of 3.2 μm×1.6 μm. As shown in FIG. 3B, the transferelectrode portion 16 and the clip electrode portion 17 have thicknessesof 450 nm and 900 nm, respectively. Since the width w2 of the clipelectrode portion 17 is smaller than the width w1 of the transferelectrode portion 16, the resistance of the clip electrode portion 17 ishigher than that of the transfer electrode portion 16. However, thethickness t2 of the clip electrode portion 17 is larger than thethickness t1 of the transfer electrode portion 16, the increase in theresistance of the gate electrode in the clip portion 17 thereby beingprevented.

The resistance of the clip electrode portion 17 decreases as thethickness thereof increases, and thus, the thicker clip electrodeportion 17 is preferable. However, if the clip electrode portion 17 istoo thick, the height of the clip portion 25 from the semiconductorsubstrate 12 is greater than that of the vertical transfer portion 24,before the flattening film 44 is formed. In this case, the distancebetween the microlens 46 and the semiconductor substrate 12 becomeslonger. Therefore, it is preferable that the thickness ratio of thetransfer electrode portion 16 and the clip electrode portion 17 is 1 to2. When the gate electrodes of the conventional solid state image device100 have the size of the surface area as shown in FIG. 3A, and when bothof the transfer electrode portion 106 and the clip electrode portion 107have the thickness of 450 nm, as shown in FIG. 3C, the resistance of thefirst gate electrode 14 decreases to about 70% of the value in theconventional element.

In the solid state image device of the invention, the shape of thesurface area of the gate electrode may be similar to that in theconventional element. Thus, since the surface area of the clip electrodeportion does not increase and part of the photoelectric transfer portionis not covered by the clip electrode portion, the incident light intothe photodiode does not decrease.

In addition, the gate electrodes of the invention consist of a singlepolysilicon layer. On the contrary, in order to reduce the resistance ofthe gate electrodes, a multilayer construction including a polysiliconfilm and tungsten silicide film may by used, for example. In that case,however, the tungsten silicide film tends to peel off from thepolysilicon film during oxidation after forming the gate electrodes.Also, there arises a problem that the polysilicon film is etched fasterthan the tungsten silicide film. According to the present invention,such problems do not arise.

Since the resistance of the gate electrode decreases, the rounding of apulse waveform voltage is suppressed. FIG. 4 shows the relationshipbetween frame transfer frequency and the rounding (potential drop) ofthe pulse waveform when the pulse waveform voltage is applied to thegate electrode shown in FIG. 3. The white dots show the result of theinvention, and the black dots show the result of the conventional art.As shown in FIG. 4, the rounding in the pulse is suppressed moresignificantly than with the conventional art as the frequency of thewaveform becomes higher. For instance, when a pulse waveform of 300 kHzis applied to the gate electrode, the voltage decreases to about 98.2%with the conventional art. However, the voltage decreases only to about99.6% according to the invention. Thus, the rounding of the drivingpulse voltage is suppressed, and the maximum amount of electric chargetransfer is improved.

The distance between the photoelectric transfer portion and themicrolens will be discussed below.

As described above referring to FIGS. 2A and 2B, the increase in thethickness of the gate electrode occurring only in the clip portion ofthe solid state image device of the invention is balanced by notproviding a light shielding metal film at the clip portion. Thus, asshown in FIG. 5B, the distance r between the photodiode 11 and themicrolens 46 in the solid state image device of the invention isapproximately equal to that of the conventional solid state imagedevice. However, if the film thickness of the whole gate electrode 50 ismade thicker to reduce the resistance of the gate electrode, thedistance between the microlens 51 and the photodiode 52disadvantageously becomes longer as shown in FIG. 5A.

FIGS. 5A and 5B show the incident light into the microlens. The brokenlines indicate parallel light. The one-dot chain lines indicatenon-parallel light.

AS shown in FIG. 5A, since the distance r is long, the curvature of themicrolens 51 needs to be smaller than that of the example shown in FIG.5B in order to adjust the focal point of the microlens 51 onto thephotodiode 52. In this case, when the incident light is non-parallel,the incident light is shielded by the light shielding metal film 53, andthe light received by the photodiode 52 thereby decreases.

According to the solid state image device 10 of the invention, on theother hand, even when the incident light is non-parallel, the lightreceived by the photodiode 11 does not decrease since the distance rdoes not increase.

FIG. 6 shows the change of the relative sensitivity with regard to the fvalue of a camera lens when the solid state image device is used in acamera. As the f value of the camera lens decreases, incident lightbecomes non-parallel. Thus, when the distance r between the microlens 51and the photodiode 52 is long as shown in FIG. 6, the relativesensitivity decreases significantly as the f value decreases, as shownby Curve (a). However, according to the invention, since the distancebetween the microlens 46 and the photodiode 11 does not change from thatin the conventional element, the reduction of the relative sensitivityis suppressed, as shown by Curve (b), even when the f value becomessmaller.

A production method for the solid state image device 10 of the inventionwill be described below.

AS shown in FIGS. 2A and 2B, boron ions are implanted in the N-typesemiconductor substrate 12, and the semiconductor substrate 12 isannealed to form a P⁻ well layer. Then, a resist pattern (not shown) isformed on the P⁻ well layer 31. Boron ions are implanted using theresist pattern as a mask to form a P-type barrier well as the firstdiffusion layer 32.

Then, phosphorus ions are implanted using the resist pattern to form thesecond diffusion layer 33 (N layer) over the entire semiconductorsubstage 10.

After removing the resist pattern, the first insulating film 37 isformed so as to cover the second diffusion layer 33 (N layer) over theentire semiconductor substrate 10.

Another resist pattern (not shown) is formed on the first insulatingfilm 37. Boron ions are implanted using the resist pattern to form thethird diffusion layer 34 (P⁺ layer). The resist pattern is removed afterthat.

FIGS. 7A through 7E show steps of forming the first and second gateelectrodes 14 and 15. In each of FIGS. 7A through 7B, a partial planview, a cross sectional view taken along Line a-a' in the partial planview, and a cross sectional view taken along Line b-b' in the partialplan view are shown.

As shown in FIG. 7A, a polysilicon film 61 having a thickness of 900 nmis deposited on the first insulating film 37. As shown in FIG. 7B, aresist pattern 62 to define the first gate electrode is formed on thepolysilicon film 61. Then, the polysilicon film 61 is etched, using theresist pattern 62 as a mask, till the insulating film 37 is exposed(FIG. 7C).

After removing the resist pattern 62, as Shown in FIG. 7D, a resistpattern 63, which covers the regions except the region becoming thetransfer electrode portion 16, is formed on part of the first insulatingfilm 37 and on part of the polysilicon film 61.

Next, the polysilicon film 61 is etched until it reaches a thickness of450 nm, using the resist pattern 63 as a mask. This process etches onlythe portion becoming the transfer electrode portion 16 in thepolysilicon film 61.

By removing the resist pattern 63, as shown in FIG. 7E, the first gateelectrode 14 is formed. The first gate electrode 14 includes thetransfer electrode portion 16 of about 450 nm thick and the clipelectrode portion 17 of about 900 nm thick.

Referring to FIGS. 2A and 2B again, after the surface of the first gateelectrode 14 is oxidized to form the second insulating film 38, thesecond gate electrode 15 is formed using the same process for the firstgate electrode 14. Using the first and second gate electrodes 14 and 15as masks, part of the first insulating film 37 is removed.

Then, after the third insulating film 39 is formed on the photoelectrictransfer portion 11 by oxidation, a resist pattern (not shown) to definethe photoelectric transfer portion 11 is formed on the third insulatingfilm 39. Phosphorus ions are implanted to form the fourth diffusionlayer 35 (N layer).

After removing the resist pattern, boron ions are implanted, using thefirst and second gate electrodes 14 and 15 as masks, to form the fifthdiffusion layer 36 (P⁺ layer) on the surface of the fourth diffusionlayer 35. The fourth insulating film 40 is formed so as to cover thesecond gate electrode 15 over the entire semiconductor substrate 12.

Next, the first light shielding metal film 41 is formed on the fourthinsulating film 40 in the vertical transfer portion 24. After thedeposition of the fifth insulating film 42 covering the first lightshielding metal film 41, the second light shielding metal film 43 isformed only in the vertical transfer portion 24. As a result, even ifthe first and second gate electrodes 14 and 15 are made thicker only inthe clip portion 25, since the light shielding metal film 41 and 43 arenot formed in the clip portion 25, the height of the clip portion 25from the semiconductor substrate 12 is not greater than that of thevertical transfer portion 24.

Then, the flattening film 44 is formed by applying an acrylic resin orthe like over the entire semiconductor substrate 12, and the colorfilter layer 45 is formed on the flattening film 44. Finally, themicrolens 46 having its center above the photodiode 11 is formed.

Although the first and second gate electrodes are made thicker in theclip portion, only one electrode may be made thicker. For instance, asshown in FIG. 8A, the thickness of a clip electrode portion 72 in theclip portion 25 of the first gate electrode 71 may be thicker than thatof a transfer electrode portion 73 in the vertical transfer portion 24,and the thickness of a clip electrode portion 76 and that of a transferelectrode portion 77 may be the same.

Alternatively, as shown in FIG. 8B, the thickness of a clip electrodeportion 82 in the clip portion of the second gate electrode 81 may bethicker than that of a transfer electrode portion 83, and the thicknessof a clip electrode portion 86 and that of a transfer electrode portion87 may be the same.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A solid state image device comprising:a pluralityof photoelectric converting elements arranged in a matrix pattern alonga first and a second direction in a semiconductor substrate; a pluralityof electric charge transfer regions for receiving electric charges fromthe photoelectric converting elements and for transferring the electriccharges toward the first direction, the electric charge transfer regionsbeing provided adjacent the plurality of photoelectric convertingelements in the semiconductor substrate and extending along the firstdirection; a plurality of gate electrodes for applying a voltage to theelectric charge transfer regions to transfer the electric charge fromthe photoelectric converting elements to the electric charge transferregions, and to transfer the electric charges toward the firstdirection, each of the gate electrodes having:a plurality of transferelectrode portions provided over the electric charge transfer regions; aplurality of clip electrode portions electrically connecting theplurality of the transfer electrode portions to each other along thesecond direction, the plurality of the clip electrode portions having agreater thickness than that of the plurality of the transfer electrodeportions such that a thickness ratio of the clip electrode portions andthe transfer electrode portions is substantially two-to-one; and lightshielding means, provided over only the electric charge transferregions, for shielding the electric charge transfer regions fromincident light, wherein a distance, between the semiconductor substrateand a top surface of the clip electrode portions, is equal to or smallerthan a distance between the semiconductor substrate and a top surface ofthe light shielding means.
 2. A solid state image device according toclaim 1, wherein the gate electrodes are made of a single layer ofpolysilicon.
 3. A solid state image device according to claim 1,whereinan increase in the thickness of the clip electrode portions issubstantially avoided by not providing the light shielding means overthe clip electrode portions.
 4. A solid state image device according toclaim 1, wherein the plurality of gate electrodes includes a pluralityof first gate electrodes and a plurality of second gate electrodes.
 5. Asolid state image device according to claim 1, further comprisingaplurality of microlenses, formed over the plurality of photoelectricconverting elements.
 6. A method of producing a solid state imagedevice, having a plurality of photoelectric converting elements arrangedin a matrix pattern along a first and a second direction in asemiconductor substrate, comprising the steps of:forming a plurality ofelectric charge transfer regions for receiving electric charges from thephotoelectric converting elements and for transferring the electriccharges toward the first direction, the electric charge transfer regionsbeing provided adjacent to the plurality of photoelectric convertingelements in the semiconductor substrate and extending along the firstdirection; and forming a plurality of gate electrodes for applying avoltage to the electric charge transfer regions to transfer the electriccharge from the photoelectric converting elements to the electric chargetransfer regions, and to transfer the electric charges toward the firstdirection, each of the gate electrodes having a plurality of transferelectrode portions, provided over the electric charge transfer regions,and a plurality of clip electrode portions electrically connecting theplurality of the transfer electrode portions to each other along thesecond direction, the plurality of the clip electrode portions having agreater thickness than that of the plurality of the transfer electrodeportions, wherein the step of forming the gate electrodes includes thesteps of:forming a polysilicon film, having a pattern of the gateelectrodes, on the semiconductor substrate; forming a resist patternover an entire surface of the semiconductor substrate, except regions ofthe polysilicon film which are intended to be the transfer electrodeportions; etching the polysilicon film, using the resist pattern as amask; forming a metal film over the entire surface of the semiconductorsubstrate; and forming a light shielding metal film by etching the metalfilm so that portions of the metal film remain only over the pluralityof electric charge transfer regions, whereby a distance between thesemiconductor substrate and a top surface of the clip electrode portionsis equal to or smaller than a distance between the semiconductorsubstrate and a top surface of the light shielding metal film.
 7. Amethod of producing a solid state image device, havinga plurality ofphotoelectric converting elements arranged in a matrix pattern along afirst and a second direction in a semiconductor substrate, comprisingthe steps of:forming a plurality of electric charge transfer regions forreceiving electric charges from the photoelectric converting elementsand for transferring the electric charges toward the first direction,the electric charge transfer regions being provided adjacent to theplurality of photoelectric converting elements in the semiconductorsubstrate and extending along the first direction; forming a firstpolysilicon film over an entire surface of a semiconductor substrate;forming a plurality of first gate electrodes by etching the firstpolysilicon film, each of the first gate electrodes having a pluralityof transfer electrode portions, provided over the electric chargetransfer regions, and a plurality of clip electrode portions,electrically connecting the plurality of the transfer electrode portionsto each other along the second direction, the plurality of the clipelectrode portions having a greater thickness than that of the pluralityof transfer electrode portions; forming a second polysilicon film overthe entire surface of a semiconductor substrate and the plurality offirst gate electrodes; forming a plurality of second gate electrodes byetching the second polysilicon film, each of the second gate electrodeshaving a plurality of transfer electrode portions, provided over theelectric charge transfer regions, and a plurality of clip electrodeportions, electrically connecting the plurality of the transferelectrode portions to each other along the second direction, theplurality of the clip electrode portions having a greater thickness thanthat of the plurality of transfer electrode portions, each of thetransfer electrode portions of the second gate electrodes partiallyoverlapping the corresponding transfer electrode portion of the secondgate electrodes; forming a metal film over the entire surface of thesemiconductor substrate; and forming a light-shielding metal film byetching the metal film, so that portions of the metal film remain onlyover the plurality of electric charge transfer regions, whereby adistance between the semiconductor substrate and a top surface of theclip electrode portions is equal to or smaller than a distance betweenthe semiconductor substrate and a top surface of the light shieldingmetal film, wherein the step of forming the plurality of first gateelectrodes includes the steps of:forming a first resist pattern on thefirst polysilicon film, the first resist pattern having a patterndefining the plurality of first gate electrodes; etching the firstpolysilicon film using the first resist pattern as a mask; forming asecond resist pattern on portions of the remaining first polysiliconfilm, namely those portions intended to be the plurality of clipelectrode portions; and etching the remaining first polysilicon film,using the second resist pattern as a mask, and wherein the step offorming the plurality of second gate electrodes includes the steps of:forming a third resist pattern on the second polysilicon film, the thirdresist pattern having a pattern defining the plurality of second gateelectrodes; etching the second polysilicon film, using the third resistpattern as a mask; forming a fourth resist pattern on portions of theremaining second polysilicon film, namely those portions intended to bethe plurality of clip electrode portions; and etching the remainingsecond polysilicon film, using the fourth resist pattern as a mask.